Digitized images are images which are represented in a digital data format. Digitized images are commonly represented in "rasterized" or "bit mapped" formats in which the value of each pixel of the image is individually represented in the data. In the case of monochrome images, each pixel may be represented by data indicating its luminance on a grey scale ranging from black (no luminance) to white (maximum luminance). Thus, a typical monochrome image data file may include one byte for each pixel, providing a range of 256 different luminance values for each pixel. In the case of "color" images, each pixel may be represented by data indicating the values of its respective red, green and blue (RGB) components. For example, an image may be represented in a "24 bit" data format which includes one byte for each of the RGB components of each pixel, thus providing a range of over 16 million different pixel values. Grey scale pixels in RGB images are typically characterized by equal red, green and blue values.
The pixels of a color image may alternatively be characterized by data representing their hue, saturation, and value (HSV) components. In the HSV system and similar systems of representation, the hue component indicates the color of the pixel, the saturation component indicates the relative amounts of color and grey-scale value of the pixel, and the value component indicates the pixel's luminance or value intensity. A variety of formulas are known for converting an RGB pixel representation to its equivalent HSV representation.
Where it is desirable to reduce the amount of data necessary to represent a color image, the pixels of an image may be represented by data associated with a color map. For example, consider an image represented in 24 bit color. While each pixel of the image may have any one of over 16 million different values, this range of color requires three bytes of storage for each pixel. However, using known processes, a histogram of the pixels of the image may be constructed to determine a smaller number of colors which are most representative of the colors of the image, and a color map including only those colors may be constructed. Subsequently, a modified image may be constructed which contains only the colors of the color map. The modified image may be represented as pixel data in the form of references to entries of the color map stored in the form of a color table. Thus if the color map contains 256 colors, each pixel of the image may be represented as a single byte reference to an entry in the color table, rather than as a three byte RGB representation.
The colors which may be displayed on known display devices such as printers and video displays vary in their dynamic ranges, i.e. in the maximum potential luminance of each color. The dynamic range of a color will depend in part on the display device. One approximation which is commonly used to determine the luminance of a pixel in an RGB image is represented by the formula L=0.3(R)+0.6(G)+0.1(B). It may be seen that in this approximation the green component G of an image contributes more luminance than the red component R, and much more luminance than the blue component B. It may be inferred from this approximation that colors in the green range of the display spectrum will have approximately twice the dynamic range of colors in the red range of the spectrum, and approximately six times the dynamic range of colors in the blue range of the spectrum.
In regard to the processing of digitized images, it is recognized that it is sometimes desirable to provide an image which represents both a set of high spatial resolution information, for example the topographical features of a land mass, and a set of low spatial resolution information which pertains to the first set of information, for example, regions of the land mass having a certain temperature. Present methods for providing such images involve providing an image formulated from a first set of information, and then replacing areas of the image with representations formulated entirely from the second set of information. Thus, using the example begun above, an image of topographical features may be produced from topographical data. Subsequently, topographical representations areas of the land mass having temperatures within a certain range may be replaced in the image by a representation indicating the temperature region of interest, for example, a uniform color. Present technology enables users of computer systems to define areas of images to be altered through the use of a pointing device such as a mouse, wherein the motions of the mouse indicate the border of an area to be altered.